8.7 TI Precision Labs - Op Amps: Noise - Measuring system noise

Hello, and welcome to the TI Precision Lab discussing intrinsic op amp noise part 7. Up to this point in the noise video series, we have learned how to predict amplifier noise output using calculation and simulation. There are two common types of test equipment used to measure noise, the oscilloscope and the spectrum analyzer.
In this video, we will discuss the theory of operation of this equipment, as well as some tips and tricks to optimize performance. Let's start by looking at the oscilloscope, which is probably the most common way that engineers measure noise. Typically, the scope is connected to the circuit output and the peak to peak noise voltage level is observed. This slide lists some tips and tricks to ensure that the observed noise reading is as accurate as possible.
The first tip relates to the type of probe connected to the scope. Most scope probes are 10x probes. This means that there is a divide by 10 attenuator inside the probe. This attenuation will reduce the noise floor by a factor of 10. So don't use this type of probe for noise measurements.
Instead, use a direct connection to the scope without any attenuation for a 10 times better noise floor. Always check the noise floor of any instrument before making noise measurements. In the case of an oscilloscope, it is common to use a BNC cap to determine the instrument's noise floor.
Many oscilloscopes have bandwidth that is much wider than your system's bandwidth. For example, you may use a 400 megahertz scope to observe the noise of a 100 kilohertz amplifier. The problem with doing this is that the scope noise floor includes a lot of extra high-frequency noise that is not relevant to your application. Most scopes have a bandwidth limiting feature that significantly reduces bandwidth and consequently improves the noise floor.
1/f noise is normally measured from 0.1 to 10 hertz. Doing this requires a dc coupled digital scope set on a very large timescale, typically one second per division. It is important to make sure that the scope is dc coupled for 1/f measurements since the typical built-in ac coupling circuit uses a 60 hertz high pass filter, which doesn't properly show flicker noise.
For broadband noise measurements on the other hand, you can use ac coupling. Ac coupling is helpful because the dc offset is eliminated, allowing for the best measurement range. Here we show a typical digital oscilloscope measuring its noise floor in three different configurations.
The worst configuration shown on the right has a noise floor of 8 millivolts peak to peak. In this case, a 10x scope probe is used, and the scope bandwidth is set to the full 400 megahertz. A significant improvement can be achieved by replacing the 10x scope probe with the direct BNC connection or 1x scope probe. Making this change effectively decreases the noise floor by a factor of 10 as shown in the center image. Notice that the vertical range changed from 10 millivolts per division to 1 millivolt per division.
The best noise floor occurs when a BNC connection is used along with the bandwidth limiting feature as shown on the left. In this example, limiting the bandwidth to 20 megahertz reduces the noise from 0.8 millivolts to 0.2 millivolts or less. This slide shows a few additional tips that can help improve the performance of your scope measurements.
First, you should avoid using the scope probe's ground lead. It can act as a loop antenna and receive extrinsic noise, giving you errors in your measurements. If possible, remove the scope probe cap, and use a direct ground connection as shown on the top right.
Note that the internal shaft on the scope probe is connected to ground. Also it is important to always measure the noise floor of your scope. One way to do this is by using a shorting cap as shown in the figure on the bottom right.
Another method is to short the end of your scope probe or measurement cable. However as was mentioned previously, your cable or scope probe can act as an antenna. Using a shorting cap will tell you the noise floor of the scope without allowing any noise pickup on the cable. It may be useful to try both methods to determine if you are picking up noise on your cable.
Once you have properly configured your oscilloscope, measuring noise is done by adjusting the time scale to match the bandwidth of your circuit. Later, we will show an example measurement of the circuit that we did hand calculation and simulation for. Let's now discuss spectrum analyzers.
The spectrum analyzer is a very useful instrument for measuring noise because it can show you the shape of the noise spectral density curve. The oscilloscope, on the other hand, does not give information as to the frequency content of your system noise. Using a spectrum analyzer can be very helpful for detecting unexpected extrinsic noise signals that are picked up. For example, you may see a spike at 60 hertz indicating that ac power line noise is being picked up.
Conceptually, the spectrum analyzer works by sweeping a band pass filter over frequency and plotting the filter's output. The width of the band pass filter is referred to as the measurement bandwidth. Averaging is also used by the instrument to improve measurement accuracy.
In the next few slides, we will cover some tradeoffs associated with changing the measurement bandwidth and averaging settings. The images above show a spectrum analyzer being used to measure signals at 67 kilohertz and 72 kilohertz. The two spectrum analyzer results are run using different measurement bandwidth settings of 150 hertz and 1,200 hertz.
The measurement that used the narrow measurement bandwidth of 150 hertz is better at resolving the discrete signals. Also the narrow measurement bandwidth reduces the noise floor because the amount of noise captured inside of the band pass filter is smaller with a narrow bandwidth. The measurement with the wide resolution bandwidth of 1,200 hertz loses information about each signal because the wider band pass filter captures both signals at once. So when making noise measurements, take care to use a measurement bandwidth that provides good resolution.
Note that decreasing measurement bandwidth will increase the sweep time, essentially trading test time for improved accuracy. In some cases for very high accuracy measurements, test times can take several hours. Thus it is not always practical to use an extremely narrow measurement bandwidth.
Another way to improve measurement accuracy is to use averaging, which combines the results of multiple noise sweeps. In order to achieve accurate results, the device conditions need to remain constant. Averaging is not good for measuring transience, but it does work very well for measuring spectral density. Averaging has the same tradeoff as with measurement bandwidth. So increasing the amount of averages for better accuracy will increase the measurement time.
In the examples above, you can see the results with no averaging on the left and the results with 49 times averaging on the right. Without averaging, the spectral density measurement shows significant variation. Using averaging, you get a more accurate overall result.
When doing noise analysis, it is useful to display measurement results as a voltage spectral density in units of nanovolts per root hertz. However, spectrum analyzers often display the measurement as decimal milliwatts or dBm. The formula above shows how to convert dBm to nanovolts per root hertz.
We will not discuss the math in detail here, but suffice it to say that we are converting the noise power delivered to the instrument's 50 ohms input impedance to a noise spectral density. In some cases, it may be useful to have a calibrated noise source to confirm that the conversion from dBm to spectral density was done accurately.
In addition to the proper configuration of oscilloscopes and spectrum analyzers, other aspects of your test setup can also have an impact on the quality of your noise measurement. First, use a well-shielded and grounded environment. Make sure that shield is grounded, and that any gaps in the shield are minimized. Copper and steel are good choices for shielding material. We often use a modified steel paint can as a shield for our noise circuits.
As mentioned before, if possible, make all circuit connections directly and with BNC cables. Use batteries or linear power supplies in order to provide the lowest noise power possible. A BNC shorting cap is useful when measuring the noise floor. Don't leave unterminated or floating inputs on your devices as these will tend to pick up extrinsic noise. Remember, the goal of this testing is to measure the intrinsic noise so these precautions are focused on eliminating sources of extrinsic noise.
Let's now apply all of these real-world techniques to the OPA672 example circuit from our hand calculations and simulations. This circuit was connected directly to an oscilloscope with a BNC cable. As previously mentioned, the direct BNC connection is better than a 10x scope probe because the noise floor is 10 times better.
The measured output noise voltage was 400 microvolts rms while the calculated result from an earlier video was 325 microvolts rms. There is some discrepancy in the measurement, which is actually typical for oscilloscope measurements. The discrepancy results from process variations in the device, as well as measurement accuracy limitations of the test equipment. In general, the agreement between measured and calculated noise should be on the order of plus or minus 20%.
If the discrepancy is quite large, first confirm that the device is connected properly and is functional. Next, make sure that the equipment is configured properly. Always confirm that the system noise floor is low enough to allow for accurate results.
Assuming that there are no functionality our equipment issues, the next thing to consider is extrinsic noise. Try to improve the shielding environment. If you still see large discrepancies after thoroughly troubleshooting the circuit, you can try noise measurement with a spectrum analyzer to get a deeper understanding of the system's noise characteristics. You may discover, for example, that switching noise at a specific frequency is significantly contributing to the noise.
Let's now use a spectrum analyzer to measure the voltage noise spectral density curve for the OPA627. For this example, we will try to reproduce the curve that's given in the OPA627 data sheet. The circuit connection is shown here.
First, note that the parallel combination of R1 and R2 is low in order to minimize the thermal noise. Also note that a large value ceramic capacitor, C1, is used to ac couple the signal into the spectrum analyzer. The spectrum analyzer input impedance and this coupling capacitor form a high pass filter with a very low cutoff frequency the 0.008 hertz. This is important for proper 1/f characterization.
The capacitive coupling is required because the op amp has a large dc offset compared to the noise level. So the dc offset would saturate the spectrum analyzer input. Note that the spectrum analyzer may have an ac coupled mode. However, the cutoff frequency is often too high for adequate 1/f measurements.
Here, we show the spectral density curve results using the circuit from the previous slide. Note that the data was collected over several ranges. For each frequency range, the spectrum analyzers measurement bandwidth is adjusted to optimize accuracy. At low frequencies, for example, the measurement bandwidth is very narrow whereas at high frequencies it is wider. This allows us to get good accuracy and also keep the measurement time reasonable.
Also note that the system noise floor was measured. Checking the noise floor is important regardless of the test equipment used. Remember, if the noise floor is higher than the signal you're trying to measure, you cannot get a valid result.
After the data is collected, you will need to make some adjustments to get the final spectral density curve. First, combine the separate frequency ranges into one curve. Second, you will notice that the curves have a strange tail at low frequencies. This a common anomaly associated with spectrum analyzers, which we will discuss in more detail in the next slide. For now, just know this data should be eliminated.
Also, you may see some extrinsic noise in your spectral density curve. In this example, you can see 60 hertz noise pickup and some harmonics of 60 hertz. Ideally, this pickup can be eliminated through proper shielding, but this is not always possible.
Finally, you will need to divide the measured results by the circuit's noise gain in order to refer the noise back to the amplifier input. This slide gives further explanation into the cause of the low frequency tail. First, keep in mind that the spectral density curve is shown on a logarithmic axis so the measurement bandwidth as a percentage of frequency is much wider at low frequency than at high frequency.
As a result, at low frequency the measurement bandwidth band pass filter captures some unwanted dc content, as well as the 1/f noise beyond the frequency that is being measured. This pushes the spectrum higher than it should be, and creates the low frequency tail. As mentioned before, this information should be eliminated. A good practice is to measure one decade lower than you need, and simply discard the low frequency points.
Here, we compare the final combine voltage spectral density curve measurement with the data sheet curve. Notice that the 1/f noise corner is different than the data sheet. This is not unusual. The 1/f noise corner changes with process variations, and the data sheet curve shows typical performance only.
Also notice that the broadband spectral density compares well between the measured result and the data sheet curve. The measured noise curve could have been improved with additional averaging and shielding. But overall, it provides an excellent depiction of the device's noise spectral density.
That concludes this video. Thank you for watching. Please try the quiz to check your understanding of this video's content. 大家好，欢迎观看 TI 高精度 实验室视频，本视频将讨论 运算放大器固有噪声的第 7 部分。 噪声视频系列 开播至今， 我们学习了 如何使用计算和仿真 预测放大器噪声输出。 噪声测量 所用的常见 测试设备有两种， 示波器和频谱 分析器。 在本视频中，我们将 讨论此设备的 工作理论，以及一些 用于优化性能的 提示和技巧。 让我们首先 来看看示波器， 这可能是工程师 最常采用的噪声测量方式。 示波器通常与 电路输出端相连， 并可观测到 峰间噪声电压水平。 这张幻灯片列有 一些提示和技巧， 以确保观测到的 噪声读数 尽可能准确。 第一个提示关于 与示波器相连的 探头类型。 大多数示波器探头 为 10x 探头。 这意味着 探头内部存在 一个除以 10 的衰减器。 该衰减将 本底噪声降低 10 倍。 所以，请勿使用此类 探头测量噪声。 反之，应使用 没有任何衰减的 示波器直接连线， 以得到好 10 倍的本底噪声。 在测量 噪声之前， 始终检查 任何仪器的本底噪声。 如果是 示波器， 则通常使用 BNC 电容器 来确定仪器的 本底噪声。 许多示波器的 带宽比您的 系统的带宽 要宽得多。 例如，您可能使用 一个 400 兆赫兹的 示波器观测 100 千赫兹 放大器的噪声。 这样做的问题是， 示波器本底噪声 含有许多与您的 应用无关的 额外 高频噪声。 大多数示波器具有 带宽限制功能， 可显著 缩小带宽， 并从而改善 本底噪声。 1/f 噪声通常在 0.1 赫兹 至 10 赫兹区间内测得。 执行此操作需要 在极大时间范围上 设置的直流 耦合数字示波器， 通常每分段 1 秒。 重要的是， 对于 1/f 测量值， 务必确保示波器与直流电路耦合， 因为典型的内置交流 耦合电路使用 60 赫兹 高通滤波器， 这将不能正确显示闪烁噪声。 对于其他方面的 宽带噪声测量， 您可以使用交流电路耦合。 交流耦合很有帮助， 因为直流偏移量已消除， 支持最佳的 测量范围。 我们在此展示了 典型的数字示波器 在三个不同的配置中 测量其本底噪声。 右侧所示的 配置最糟， 峰间值为 8 毫伏本底噪声。 在这种情况下， 使用了 10 倍示波器探头， 并且示波器带宽 已设为全频 400 兆赫兹。 通过将 10 倍 示波器探头替换为 使用 BNC 直接 连接 1 倍探头， 取得了显著改善。 此变更 将本地噪声 有效降低了 10 倍，如中间图像所示。 请注意，垂直区间 从 10 毫伏/分段 变为 1 毫伏/分段。 当使用 BNC 连接和 带宽限制功能时， 最佳本底噪声出现， 如左侧所示。 在此示例中， 将带宽限制在 20 兆赫兹 将使噪声 从 0.8 毫伏 减至 0.2 毫伏或更低。 此幻灯片展示了 有助于改善示波器 测量性能的 一些附加 提示。 首先，您应避免使用 示波器探头的接地导线。 它可作为环路天线 并接收非固有噪声， 使您产生 测量误差。 如有可能， 拆除示波器探头电容， 并使用直接 接地连接， 如右上方所示。 请注意， 示波器探头中的内部轴 已接地。 同样重要的是， 始终测量 示波器的噪声本底。 为此可采用的一种方法是 使用短接电容， 如右下图 所示。 另一方法是 短接示波器探头端点 或测量线缆。 然而，如先前提到， 您的缆线 或示波器探头 可以用作天线。 使用短接电容 将告诉您 示波器的本底噪声， 而不会在线缆上 拾取任何噪声。 尝试一并使用两种方法 可能会有助于确定 您是否在拾取缆线上的噪声。 一旦正确 配置示波器， 将时间标度 调整为与电路带宽相匹配， 从而完成 噪声测量。 稍后，我们将展示 我们曾为其进行手算和仿真的 电路的 测量示例。 现在让我们讨论 频谱分析仪。 频谱分析仪是 非常有用的 噪声测量仪器， 因为它能够 向您展示噪声频谱 密度曲线的形状。 另一方面， 示波器 不提供 与您的系统噪声的 频率内容有关的信息。 使用频谱 分析仪非常 有助于检测 意外拾取的 非固有噪声信号。 例如，您可能在 60 赫兹 见到尖峰， 指示此时拾取了 交流电源线噪声。 从概念上而言， 频谱分析仪 工作原理是扫描 过频的带通滤波器 并绘制 滤波器的输出。 带通滤波器的 宽度 称作 测量带宽。 此外，该仪器 还使用平均 来改善测量精度。 在接下来的几张幻灯片中， 我们将介绍与更改 测量带宽和 平均设置值 有关的一些折衷。 上图展示了 用于测量 67 千赫兹与 72 千赫兹处 信号的频谱分析仪。 两个频谱 分析仪结果 均使用不同的 测量带宽设置 150 赫兹与 1,200 赫兹运行。 使用窄测量 带宽 150 赫兹的测量 在解析 离散信号上 表现更佳。 此外，窄测量 带宽 将减少本底噪声， 因为在采用窄带宽后， 带通滤波器 内部捕获的 噪声量变小。 采用 1,200 赫兹 宽分辨率带宽的 测量丢失了每个 信号的相关信息， 因为较宽的带通滤波器 将立即捕获所有这 两种信号。 噪声 测量期间， 请注意选用 提供最佳分辨率的 测量带宽。 请注意， 降低测量带宽 将增加扫描时间， 主要是以测试时间为代价 换取更高的精度。 在有些极高 精度测量的情况下， 测试时间 可能需要数小时。 因此，使用极窄的 测量带宽 并不是总切实可行。 另一种改善 测量精度的方法是 使用平均计算，该方法将 多个噪声扫描的结果 组合在一起。 为了实现 精确的结果， 器件条件 需要保持恒定。 平均计算不是测量瞬变 过程的好方法， 但是它对于测量频谱密度 确实是极好的方法。 平均计算采用与 测量带宽相同的折衷手段。 提高平均计算的量 以换取更高的精度 将提高 测量时间。 在上例中， 您可以见到 左侧是未采用 平均计算的结果， 在右侧是经过 49 次 平均计算得到的结果。 如不采用平均计算， 频谱密度测量 将出现显著偏差。 如采用平均计算，您将获得 更为准确的总体结果。 在进行噪声分析时， 将测量结果显示为 以纳伏/根赫兹为单位的 电压频谱密度， 将有所帮助。 但是，频谱 分析仪 以十进制的毫瓦 或 dBm 显示测量值。 以上公式显示 如何将 dBm 转化为纳伏/根赫兹。 我们在此不详细 讨论数学， 但是可以这么说， 我们将传递给 仪器的 50 欧姆输入 阻抗的噪声功率 已转化为 噪声频谱密度。 在有些情况下， 拥有经校准的噪声源 可能有助于 确认从 dBm 转化为 频谱密度的 工作已准确完成。 除正确配置示波器 和频谱分析仪外， 测试设置的 其他方面 也可影响 噪声测量的 质量。 第一，使用屏蔽良好 且接地的环境。 确保 屏蔽件已接地， 并且已最大程度减少 屏蔽件中的任何缝隙。 铜和铁是很好的 屏蔽材料选择。 我们常常使用 修改后的铁防锈漆 作为我们的噪声 电路屏蔽件。 如上 所提到，尽可能 确保所有电路直接连接 并使用 BNC 缆线。 使用电池或 线性电源， 以使提供的 噪声功率尽可能低。 在测量本低噪声时， BNC 短接电容很有用。 器件输入端 必须具有终端接头且不得浮动。 因为这些往往将 拾取非固有噪声。 请记住，此测试的 目标是 测量 固有噪声， 所有这些预防措施 侧重于消除 非固有噪声源。 让我们现在将所有 这些现实世界中的技术 应用到来自我们的 手算和仿真的 OPA672 示例电路中。 此电路 通过 BNC 电缆 直接连接至 示波器。 如先前提到， 直接连接 BNC 要优于 10 倍示波器探头， 因为本底噪声会 好 10 倍。 测得的输出 噪声电压为 400 微伏 RMS， 而先前视频的 计算结果 为 325 微伏 rms。 测量值上 存在一些差异， 这实际上是典型的 示波器测量情况。 此差异是 因为存在 器件的流程变化， 并且测试设备 存在测量 精度限制。 一般而言， 测得噪声和 算得噪声之间的相符度 应在 +/-20% 的 阶。 如果差异 特别大， 首先确认器件 确已正确连接 且功能正常。 下一步，确保 设备 已正确配置。 始终确认系统 本底噪声低至足以 获得准确的结果。 假定我们的设备 没有任何功能性问题， 那么接下来要考虑 非固有噪声。 尝试改善 屏蔽环境。 在彻底排除 电路故障后， 如果您仍发现 存在大差异， 则您可以尝试使用 频谱分析仪测量噪声， 以更深一步 了解系统的噪声 特性。 您可能发现，例如， 特定频率处的 开关噪声 对噪声的 贡献显著。 现在让我们使用 频谱分析仪 测量 OPA627 的 电压噪声 频谱密度曲线。 例如，我们将尝试重现 在 OPA627 产品说明书中 给出的曲线。 电路连接 如此处所示。 首先，请注意 R1 和 R2 的 的并联组合较低 以最大程度 减少热噪声。 另请注意， 大值陶瓷电容器 C1 用于将信号通过交流电路耦合 至频谱分析仪。 频谱分析仪输入阻抗 以及该耦合电容器 构成高通 滤波器， 且具有 0.008 赫兹的 极低截止频率。 这对于正确的 1/f 表征 非常重要。 需要电容耦合 是因为， 与噪声水平相比， 运算放大器的 直流偏移量大。 直流偏移将使 频谱分析仪输入饱和。 请注意，频谱分析仪 可能具有交流电路耦合模式。 但是，截止频率 往往过高， 以致无法 充分测量 1/f 噪声。 我们在此使用 上一张幻灯片的电路， 展示了 频谱密度曲线的结果。 请注意，这些数据 自多个区间收集。 对于每个频率 区间，频谱 分析仪 测量带宽 已为优化性能而经过调整。 例如，在低频， 测量带宽 非常狭窄，而在 高频，它变得较宽。 这让我们能够 实现高精度， 并且还能使测量时间 保持在合理水平。 另请注意， 系统本底噪声已测量。 检查本底噪声 很重要， 无论使用何种测试设备 均是如此。 请记住，如果本底噪声高于 您尝试测量的信号， 您将无法获得 有效的结果。 在数据收集后， 您将需要 做些调整才能 得到最终的频谱密度 曲线。 第一，将单独的 频率区间合并为 一条曲线。 第二，您将注意到 曲线在低频处具有 奇怪的尾部。 这是与频谱分析仪 相关的常见异常情况， 我们将在下一幻灯片中 更为详细地讨论。 现在，只需了解 应清除该数据即可。 另外，您可能见到 在您的频谱密度曲线中 有一些非固有噪声。 在此示例中， 您能够见到 60 赫兹噪声拾取 以及有些 60 赫兹的谐波。 理想情况下， 通过正确屏蔽 可消除该拾取，但并非 始终可以实现。 最终，您需要 将测得结果 除以电路的噪声增益 以将噪声引用回到 放大器输入。 此幻灯片给出了 对低频尾部 产生原因的 进一步解释。 第一，请记住， 频谱密度曲线 显示在对数轴 ，因此以频率 百分比表示的 测量带宽 在低频时比 在高频时要宽得多。 结果，在低频， 测量带宽 带通滤波器将捕获 有些无用的直流内容， 以及超过 测量频率的 1/f 噪声。 这将推动频谱高于 其应有的范围， 并产生 低频尾部。 如前所述， 应消除 此信息。 有个好的做法是测量低于 您所需信息一个十年期的频率， 并且仅抛弃 低频点。 在此，我们将比较 最终组合电压频谱密度曲线 测量值与 产品说明书所载曲线。 请注意， 1/f 噪声转角点 与产品说明书所载内容有所不同。 这并非个别情况。 1/f 噪声转角点 随流程变化而变， 并且产品说明书所载曲线 仅展示典型性能。 另请注意，测得结果和 产品说明书曲线之间的 宽带频谱密度 对比情况良好。 测得噪声 曲线可能 已通过额外的平均运算 与屏蔽获得改善。 但总体而言，它会提供 绝佳的 器件噪声 频谱密度描述。 本视频到此结束。 谢谢观看。 请尝试完成测验以 检查您对本视频 内容的理解。

Description

March 23, 2015

This is the seventh of nine videos in the TI Precision Labs – Op Amps curriculum that addresses operational amplifier noise. Up to this point in the noise training video series we have learned how to predict amplifier noise output using calculation and simulation. In this training we will cover techniques for measuring noise. There are two common types of test equipment that are used to measure noise: the oscilloscope and the spectrum analyzer. In this video we will discuss the theory of operation of this equipment, as well as some tips and tricks to optimize performance.

After watching the video, reinforce your learning with the following bonus content: